7 Let the Titrations Begin

harris_9e_ch07_145-160.indd Page 145 1/5/15 1:27 PM f-479 /204/WHF00272/work/indd 7 Let the Titrations Begin TITRATION ON MARS Robotic arm of Phoenix Mars Lander scoops up soil for chemical analysis on Mars. [NASA/JPL- Caltech/University of Arizona/Texas A&M University.] In 2008, Professor Sam Kounaves and his students at Tufts University had the thrill of a life- time as their Wet Chemistry Laboratory aboard the Phoenix Mars Lander returned a stream of information about the ionic composition of Martian soil scooped up by a robotic arm. The arm delivered ~1 gram of soil through a sieve into a “beaker” fi tted with a suite of electro- chemical sensors described in Chapter 15. Aqueous solution added to the beaker shown in Box 15-3 leached soluble salts from the soil, while sensors measured ions appearing in the liquid. Unlike other ions, sulfate was measured by a precipitation titration with Ba21: S 21 2 FreemanBaCl2(s) Ba 1 2Cl 22 21 S SO4 1 Ba BaSO4(s) As solid BaCl2 from a reagent canister slowly dissolved in the aqueous liquid, BaSO4 precipitated. In Problem 7-21, we see that one sensor showed a low level of Ba21 until 22 enough reagent had been added to react with all SO4 . Another sensor found steadily increas- 2 ing Cl as BaCl2 dissolved. The end point of the titration is marked by a sudden increase of 21 22 Ba when the last SO4 has precipitated and BaCl2 continues to dissolve. The increase in 2 Cl from the beginning of the titration up to the end point tells how much BaCl2 was required 22 to consume SO4 . Titrations of two soil samples in two cells found ,1.3 (60.5) wt% sulfate 1 W.H.in the soil. Other evidence suggests that the sulfate is mostly from MgSO4. rocedures in which we measure the volume of reagent needed to react with analyte are Pcalled volumetric analysis. In this chapter, we discuss principles that apply to all volumetric procedures and then focus on precipitation titrations. Acid-base, oxidation-reduction, complex formation, and spectrophotometric titrations are discussed later in the book in their respective chapters. 7-1 Titrations In a titration, increments of reagent solution—the titrant—are added to analyte until their reaction is complete. From the quantity of titrant required, we can calculate the quantity of analyte that must have been present. Titrant is usually delivered from a buret (Figure 7-1). CHAPTER 7 Let the Titrations Begin 145 harris_9e_ch07_145-160.indd Page 146 1/5/15 1:27 PM f-479 /204/WHF00272/work/indd The principal requirements for a titration reaction are that it have a large equilibrium constant and proceed rapidly. That is, each increment of titrant should be completely and quickly consumed by analyte until the analyte is used up. Common titrations rely on acid- base, oxidation-reduction, complex formation, or precipitation reactions. The equivalence point occurs when the quantity of added titrant is the exact amount necessary for stoichiometric reaction with the analyte. For example, 5 mol of oxalic acid react with 2 mol of permanganate in hot acidic solution: O O ™ ™ ® ® ® Ϫ B ϩ 2ϩϩ Reaction 7-1 is an oxidation-reduction reaction. 5HO C C OH 2MnO4 6H 10CO2 2Mn 8H2O (7-1) Please study Appendix D if you need to relearn how to balance Reaction 7-1. Analyte Titrant Oxalic acid Permanganate colorless colorless colorless purple If the unknown contains 5.000 mmol of oxalic acid, the equivalence point is reached when 2 2.000 mmol of MnO 4 have been added. The equivalence point is the ideal (theoretical) result we seek in a titration. What we actu- ally measure is the end point, which is marked by a sudden change in a physical property of the solution. In Reaction 7-1, a convenient end point is the abrupt appearance of the purple color of permanganate in the fl ask. Prior to the equivalence point, all permanganate is con- Level of titrant sumed by oxalic acid, and the titration solution remains colorless. After the equivalence point, 2 unreacted MnO4 accumulates until there is enough to see. The fi rst trace of purple color is the end point. The better your eyes, the closer will be your measured end point to the true equivalence point. Here, the end point cannot exactly equal the equivalence point, because extra Buret 2 clamp MnO4, beyond that needed to react with oxalic acid, is required to exhibit purple color. Methods for determining when the analyte has been consumed include (1) detecting a Buret sudden change in the voltage or current between a pair of electrodes (Figure 7-5), (2) observing an indicator color change (Color Plate 2), and (3) monitoring absorption of light (Figure 18-11). An indicator is a compound with a physical property (usually color) that changes abruptly near the equivalence point. The change is caused by the disappearance of analyte or the appearance of excess titrant. The difference between the end point and the equivalence point is an inescapable titration error. By choosing a physical property whose change is easily observed (such as pH or Stopcock the color of an indicator), the end point can be very close to the equivalence point. We esti- mate the titration error with a blank titration, in which we carry out the same procedure without analyte. For example,Freeman we can titrate a solution containing no oxalic acid to see how 2 much MnO 4 is needed to produce observable purple color. We then subtract this volume of 2 MnO 4 from the volume observed in the analytical titration. The validity of an analytical result depends on knowing the amount of one of the reac- tants used. If a titrant is prepared by dissolving a weighed amount of pure reagent in a known Flask volume of solution, its concentration can be calculated. We call such a reagent a primary standard, because it is pure enough to be weighed and used directly. A primary standard should be 99.9% pure, or better. It should not decompose under ordinary storage, and it should be stable when dried by heat or vacuum, because drying is required to remove traces Solution of water adsorbed from the atmosphere. The highest quality, commercially available, primary of analyte Magnetic stirring standard pure materials, such as As2O3 (arsenic(III) oxide), CaCO3, Hg metal, and Ni metal, W.H.bar have purities certifi ed to within 60.05%. Primary standards for many elements are given in Appendix K. Box 7-1 discusses reagent purity. Box 3-2 describes certifi ed reference materials that allow laboratories to test the accuracy of their procedures. Many reagents used as titrants, such as HCl, are not available as primary standards. Instead, we prepare titrant with approximately the desired concentration and use it to titrate a primary standard. By this procedure, called standardization, we determine the concentration Magnetic of titrant. We then say that the titrant is a standard solution. The validity of the analytical stirrer result ultimately depends on knowing the composition of a primary standard. Sodium oxalate (Na C O ) is a commercially available primary standard for generating oxalic acid to stan- FIGURE 7-1 Typical setup for a titration. The 2 2 4 analyte is contained in the ﬂ ask, and the titrant dardize permanganate in Reaction 7-1. is in the buret. The stirring bar is a magnet In a direct titration, we add titrant to analyte until the reaction is complete. Occasion- coated with Teﬂ on, which is inert to most ally, we perform a back titration, in which we add a known excess of one standard reagent solutions. The bar is spun by a rotating magnet to the analyte. Then we titrate the excess reagent with a second standard reagent. A back inside the stirring motor. titration is useful when its end point is clearer than the end point of the direct titration, or 146 CHAPTER 7 Let the Titrations Begin harris_9e_ch07_145-160.indd Page 147 1/5/15 1:27 PM f-479 /204/WHF00272/work/indd BOX 7-1 Reagent Chemicals and Primary Standards Chemicals are sold in many grades of purity. For analytical analytical chemistry, carry designations such as “chemically pure” chemi stry, we usually use reagent-grade chemicals meeting (CP), “practical,” “purifi ed,” or “technical.” purity requirements set by the American Chemical Society (ACS) A few chemicals are sold in high enough purity to be primary Committee on Analytical Reagents.2 Sometimes “reagent grade” standard grade. Whereas reagent-grade potassium dichromate has a simply meets purity standards set by the manufacturer. An actual lot lot assay of $99.0%, primary standard grade K2Cr2O7 must be in analysis for specifi ed impurities should appear on the reagent bottle. the range 99.95–100.05%. Besides high purity, a key quality of For example, here is a lot analysis of zinc sulfate: primary standards is that they are indefi nitely stable. For trace analysis (analysis of species at ppm and lower levels), impurities in reagent chemicals must be extremely low. For this ZnSO4 ACS Reagent Lot Analysis: purpose, we use very-high-purity, expensive grades of acids such as Assay: 100.6% Fe: 0.000 5% Ca: 0.001% “trace metal grade” HNO3 or HCl to dissolve samples. We must pay Insoluble matter: careful attention to reagents and vessels whose impurity levels could 0.002% Pb: 0.002 8% Mg: 0.000 3% be greater than the quantity of analyte we seek to measure. pH of 5% solution The following steps help you to protect the purity of chemical at 25°C: 5.6 Mn: 0.6 ppm K: 0.002% reagents: Ammonium: • Avoid putting a spatula into a bottle.

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